Work Toward TCL Parity Skill

Step-by-step process for implementing missing TCL behaviors to achieve parity with standard TCL. Use this when a feature is partially implemented and needs additional behaviors to match TCL.

When to Use

• A builtin exists but is missing specific behaviors
• Documentation lists features as "Not implemented"
• Tests pass but behavior differs from TCL oracle

Working Process

1. Review Last Commit

Before starting any work:

git show HEAD

Understand the current state and any handoff notes from your colleague.

2. Identify Missing Behaviors

Review the documentation file for the feature:

cat docs/builtin-<feature>.md

Look for entries marked as "Not implemented" or features that differ from TCL.

3. Explore Oracle Behavior

Test expected behavior against the reference TCL interpreter interactively:

tclsh << 'EOF'
# Test code here
EOF

Or use the oracle wrapper:

echo '<tcl code>' | bin/oracle

Key areas to explore:

• Basic functionality
• Edge cases
• Error conditions and messages
• Interaction with other features (e.g., upvar, nested calls)

4. Create Test Cases First

Create or update testcases/<feature>.html with test cases for the missing behaviors.

Use the correct test case structure:

<test-case name="descriptive name">
  <script>
    tcl code here
  </script>
  <return>TCL_OK</return>
  <error></error>
  <stdout>expected output</stdout>
  <stderr></stderr>
  <exit-code>0</exit-code>
</test-case>

For error cases:

• Set <return>TCL_ERROR</return>
• Set <error>expected error message</error>
• Set <exit-code>1</exit-code>
• Omit <stdout> tag entirely (do not include empty <stdout></stdout>)

5. Verify Tests Against Oracle

bin/harness run --host oracle testcases/<feature>.html

All tests must pass against the oracle before implementation. Fix any test expectation mismatches.

6. Commit Tests

Commit the tests separately before implementing:

git add testcases/<feature>.html
git commit -m "Add test cases for <feature> <specific behaviors>"

This ensures tests are preserved even if implementation is incomplete.

7. Implement the Feature

Now implement to make the tests pass. Common patterns:

Modifying Function Signatures

When adding return values or parameters:

1. Update src/internal.h declarations
2. Update implementation in src/<file>.c
3. Update all call sites

Adding Host Interface Functions

If new host operations are needed: you first MUST STOP and ASK THE USER whether the extension is appropriate.

Extending the host interface is a measure of last resort: we want to keep as much logic in the C core as possible.

First think again whether you could implement the feature without extending the host interface.

If you have been granted permission to extend the host interface, follow these steps:

1. Add to src/feather.h in appropriate ops struct
2. Add declaration to src/host.h
3. Add C wrapper in callbacks.c
4. Add to ops initialization in src/host.c
5. Implement in Go (interp_callbacks.go)
6. Implement in JS (js/feather.js)

8. Build and Test Iteratively

# Build
mise build

# Run specific tests
bin/harness run --host bin/feather-tester testcases/<feature>.html

# When tests pass, run full regression
mise test

# Run JS/WASM tests
mise test:js

Fix failures one at a time. Common issues:

• Wrong argument counts in trace callbacks
• Missing cases in switch statements
• Error message format differences

9. Commit Implementation

git add -A
git commit -m "Implement <specific behaviors> for <feature>

<Detailed description of what was implemented>
<Key implementation decisions>

All N tests pass in both Go and JS/WASM hosts."

10. Update Documentation

Update docs/builtin-<feature>.md:

• Change "Not implemented" to "Implemented"
• Add implementation notes sections if needed
• Document any remaining limitations

Update docs/index.md if the feature status changed significantly.

11. Final Commit

git add docs/
git commit -m "Update <feature> documentation to reflect implementation"

Key Files

Common Patterns

Error Propagation

Different trace types handle errors differently:

• Variable read/write: wrap as can't read/set "varname": <error>
• Variable unset: ignore errors
• Command traces: ignore errors, restore result
• Execution traces: propagate directly

Nested Propagation

When behavior must propagate through nested calls:

1. Create a variable in the interpreter in the ::tcl namespace
2. Save/restore the previous value when entering/exiting
3. Check the global in inner functions to continue the behavior

Working Around Feather Limitations

If tests use TCL features Feather doesn't support (e.g., lindex $list end):

1. Verify the test passes against oracle first
2. Rewrite using supported constructs (e.g., lindex $list [expr {$len - 1}])
3. Re-verify against oracle
4. Document the workaround in commit message

Example Session Flow

1. User: "Implement enterstep/leavestep traces"
2. Explore: tclsh to understand behavior
3. Create: testcases/trace-step.html with 15 test cases
4. Verify: bin/harness run --host oracle testcases/trace-step.html
5. Commit: Tests only
6. Implement: Add stepped eval functions, modify proc invocation
7. Test: bin/harness run --host bin/feather-tester testcases/trace-step.html
8. Fix: Handle leavestep arguments, workaround lindex end
9. Verify: mise test and mise test:js
10. Commit: Implementation
11. Update: docs/builtin-trace.md and docs/index.md
12. Commit: Documentation